Certain medical conditions, traumas, and surgeries result in a patient's heart not being able to maintain adequate blood pressure to perfuse the patient's organ systems and tissues. Medical advances in circulation assist devices focus on this area wherein the heart or part of the heart is not able to contract and relax with the force necessary to maintain blood flow and pressure. These devices have been referred to ventricular assist devices, or “VAD.” One specific type of VAD is a left ventricular assist device, or “LVAD.”
In the simplest of terms, the right ventricle of the heart receives blood from the right atrium. The blood from the right atrium has come from the body and is thus deoxygenated. Once delivered from the right atrium to the right ventricle, the blood is pumped by the right ventricle through the three-leafed pulmonary valve and into the pulmonary artery. The blood has yet to be oxygenated in the lungs and so the pulmonary artery is the only artery that carries deoxygenated blood. After the blood has been oxygenated in the lungs, it is delivered to the left atrium. From the left atrium, the blood is pumped into the left ventricle where, in the healthy heart, it is pumped through the three-leafed aortic valve to the aorta to be distributed throughout the body. Circulation assist devices are most commonly indicated in left ventricular dysfunction, a dysfunction that does not allow the necessary perfusion of the body with oxygenated blood.
The intra-aortic balloon pump (“IABP”) is the most commonly used LVAD to enhance a patient's cardiac output when the patient's heart fails to maintain a sufficient arterial blood pressure. An IABP, however, cannot create blood pressure when there is no left ventricle function, or during severe arrhythmia or fibrillation. U.S. Pat. No. 6,190,304 Downey discloses an IABP that is directed to creating additional blood pressure, but it does not indicate whether it can raise blood pressure from zero. Further, IABPs, in general, typically operate on a pulsate basis, and do not maintain constant blood pressure throughout the heart cycle. Moreover, IABPs typically use helium as the drive gas for the balloon and some require complex control systems. Vascular complications associated with IABPs can include perforation of the aortic wall, aortoiliac dissection, limb ischemic complications, etc.
Most LVAD surgically implanted pumps designed are as a “bridge to transplant” or “bridge to recovery.” Because most of these pumps require surgical implantation, the patient must be a good surgical candidate. Many heart patients take thrombolytics or anti-coagulants and surgery is risky due to the potential for uncontrollable bleeding. In addition, these devices require considerable time to implant during a surgical procedure, and thus are not suitable for emergency situations, or situations where a patient is not in need of an assist device for any extended period.
Besides the IABPs, a number of other types surgical implanted LVAD pumps are currently available. Examples of these pumps include the Heartmate®, Thoratec®, Novacor® and Abiomed®. Axial flow LVAD pumps are available, but also require surgical implantation through the chest wall. Most of the axial flow pumps are in trial stages at this time, including the MicroMed DeBakey VAD® and the Jarvik 2000®.
A percutaneous axial flow pump was introduced in 1988 and commercially released in Germany in 1996. Called the Hemopump®, it consisted of an Archimedes screw driven by a cable within a cannula. An invention similar to the Hemopump is described in U.S. Pat. No. 5,092,844 (Schwartz et al), which describes a variety of pumping mechanisms in a sheath. Complications with the Hemopump may occur if the drive cable fractures or the cannula is expelled from the ventricle. The Hemopump was available in three variants, a 14 French version for percutaneous insertion, a 21 French version for introduction via graft anastomosis to the femoral artery and a 26 French version for direct insertion into the ascending aorta. The 21 French size Hemopump, however, could not always be successfully inserted through the femoral artery due to its size and a 14 French size Hemopump could only pump at rates up to 1.5 liters per minute.
Another percutaneous blood pump is described in U.S. Pat. No. 5,749,855 (Reitan), which uses an open propeller without a surrounding cylindrical pump housing. It requires a catheter with a lattice or bars to protect the aorta and the impeller. This device is being used only in the aorta and not in the left ventricle or the pulmonary artery.
There is current need for a blood circulation assist device that:                1. can increase cardiac output;        2. increase blood pressure when there is little or no ventricular function;        3. can maintain blood pressure throughout the heart cycle;        4. uses a simple drive and control system;        5. can be placed percutaneously via a guidewire in a very compact delivery package;        6. can be quickly placed and operated;        7. provides sufficient pumping capacity; and        8. does not traumatize the surrounding tissues.        